Disk recorders of either magnetic or optical type employ transducer positioning systems. Such transducer positioning systems often use a velocity servo loop for long transducer radial motions, termed seeks. Upon reaching a target track after leaving an origin track, the operation of the positioning systems switches from a velocity seek mode to a track following mode. In some instances, digital techniques are employed in the seeks while analog techniques are employed in the track following. Such a track following control may be favorably compared to (stop-lock) positioning control in other servo positioning applications. In a subsequent seek operation, the track following position mode is interrupted to return to a velocity loop control or a second positioning loop of another form, particularly when optical disk reliable track counting has been a continuing problem. One of the difficulties arises from the eccentricity in disk rotation combined with one micron intertrack spacing. Such so-called repeatable "runout" or "eccentricity of rotation" causes false direction of motion indications between a track being crossed and the absolute motion of the transducer. This problem increases at low radial speeds of the transducer. Accordingly, it is desired to provide for more reliable track seeking and control during the velocity portion of a track seek operation.
Another critical aspect of velocity servo control is the generation of a reliable velocity or speed indicating signal. The speed indicating signal often is derived from counting tracks as they are crossed, therefore, its reliability is dependent upon the reliability of the detection of track crossings by the transducer being moved. It is also desired to digitize portions of the track seeking and following system. Various techniques for enhancing the reliability of the positioning servo system while employing digital techniques are desired.
Optical recorders having the goal of high performance at low cost employ high performance servo positioning systems. The mechanical construction used in connection with these positioning systems is a so-called "fine servo," or actuator (also termed a secondary headarm or topping servo), is carried on a primary transducer or head-carrying arm which is radially movable by a so-called coarse actuator. The fine actuator is movably mounted by the coarse actuator to be servoed to a referenced position carried on the coarse actuator. Typically, the fine actuator has a high-frequency response characteristic for providing rapid and short-distance positioning motions of the transducer with respect to a track being followed or moving from one track to a second or target track that is relatively close to the one or origin track. The coarse servo which positions a relatively large mass primary head-carrying arm typically has low frequency characteristics for handling the longer moves. For optimizing the relationship for top performance between such fine and coarse actuators, positioning servo systems provide for the coarse actuator to always follow the fine actuator. Such arrangements have been colloquially called "piggy-back" carriage servo systems.